This invention relates generally to methods and apparatus for tomosynthetic volume reconstruction (creating a volumetric image from projections) and more particularly to methods and apparatus for reconstructing volumes from tomosynthesis projection images such that the intensities in the reconstructed volume correspond quantitatively to a number of selected material classes.
In at least one known medical imaging method, radiological interpretation of two-dimensional projection radiographs of a breast volume is used in combination with image processing techniques that aim solely at optimizing the quality of the displayed projection images. In standard mammography (i.e., standard projection radiography of the breast) the radiologist has only limited information available for projection image interpretation. Structures that exist at distinct physical locations in the imaged breast can appear superimposed on the projection radiograph, thereby hiding lesions, or the superimposition of normal structures can mimic the appearance of cancerous lesions. In addition, there is no absolute scale of the gray values within the image, so that a radiologist must rely on his or her experience to interpret projection images and to attribute projection image content to locally varying tissue composition and/or thickness.
Qualitatively, brighter regions of the projection image correspond to detected x-ray beams that passed through an increased tissue thickness, or relatively higher fraction of glandular tissue. Darker regions correspond to detected x-ray beams which passed through a reduced tissue thickness, or a relatively higher fraction of fatty tissue. Furthermore, the appearance of the projection image depends on the x-ray technique that was used in the image acquisition, as well as any image processing steps.
Tomosynthesis is a 3D x-ray imaging technique in which a 3D volumetric image is reconstructed from a few projection radiographs of an object, where the projection radiographs are typically acquired for different x-ray tube positions over a small angular range relative to the imaged object. The reconstructed 3D volumetric image provides information about the three-dimensional location, shape, and extent of structures within the imaged breast. Thus, reconstructed volumetric images in tomosynthesis may alleviate problems of interpreting overlapping tissue. However, while current reconstruction and image processing approaches aim at obtaining a good image quality for the reconstructed 3D volumetric image, maybe targeting a specific display mode, an absolute quantitative relationship between voxel intensities (in the 3D reconstructed volumetric image) and material composition (e.g., of breast tissue) is not currently available.